Antiulcer drugs having both functions of suppression of gastric acid secretion and protection of the gastric mucosa have been demanded. Histamine H.sub.2 receptor blocking agents typically including cimetidine are drugs which suppress gastric acid secretion. However, these drugs have no action of protecting the gastric mucosa and, in addition, show unfavorable side effects on the central nervous system and, therefore, are not deemed to be sufficient for prevention and treatment of ulcers.
[H.sup.+ -K.sup.+ ]ATPase inhibitors typically including omeprazole powerfully suppress gastric acid secretion but are known to cause anacidity. Besides, they are labile against acids and are susceptible to decomposition by gastric acid.
Accordingly, there has been a need to develop an antiulcer agent which exhibits both suppression of gastric acid secretion and protection of the gastric mucosa in a good balance and is effective on various ulcers, of low toxicity, and stable to gastric acid.